The 3rd Generation Partnership Project (3GPP) has been standardizing Cellular Internet of Things (CIoT). CIoT covered by 3GPP includes Long Term Evolution enhanced Machine to Machine (LTE eMTC) and Narrowband IoT (NB-IoT). The characteristic features of LTE eMTC and NB-IoT include ultra-low User Equipment (UE) power consumption, a large number of devices per cell, narrowband spectrum, and extended coverage. In LTE eMTC (Category M), UE Radio Frequency (RF) bandwidth is defined as 1.4 MHz. Meanwhile, in NB-IoT, it is assumed that downlink and uplink peak rates are 200 kbps or 144 kbps, and UE RF bandwidth is about 200 kHz (effective bandwidth is 180 kHz) in both uplink and downlink for further cost optimization, low power consumption, and coverage extension.
Non Patent Literature 1 describes several communication architecture solutions for infrequent small data transmission in the NB-IoT. These solutions include an architecture for data transmission through the control plane (i.e., Solution 2) and an architecture for data transmission through the user plane (i.e., Solution 18) involving suspension and resumption of an RRC connection. In Non Patent Literature 1, support of the solution 2 is mandatory for both the UE and the network, while support of the solution 18 is optional for both the UE and the network.
The solution 2 is based on the lightweight Core Network (CN) architecture for CIoT. In the lightweight CN architecture, in consideration of typical use cases of CIoT devices, the core network only supports a limited number of functions, compared to the CN entities in the existing LTE (i.e., Mobility Management Entity (MME), Serving Gateway (S-GW), and Packet Data Network Gateway (P-GW)). FIG. 1 shows a network architecture for CIoT in a non-roaming case.
CIoT Serving Gateway Node (C-SGN) is a new logical network entity. The C-SGN is a CN node having both the control plane (CP) and the user plane (UP). The C-SGN provides a limited Mobility Management (MM) procedure for CIoT devices, a small data transmission procedure, a security procedure for small data transmission, and a termination of an SGi interface for the non-roaming case. The P-GW function may be separated from the C-SGN. In this case, an S5 interface is used between the C-SGN and the P-GW. In a roaming case, the C-SGN provides an S8 interface.
The S1-lite interface is an optimized version of S1-C(S1-MME). The S1-lite interface supports S1 Application Protocol (S1AP) messages and information elements (IEs) for CIoT procedures, and also supports optimized security procedures. For efficient small data transmission, user data is delivered through the SLAP layer.
Specifically, in the Mobile Originated (MO) small data transmission for the non-roaming case, the UE transmits an uplink Non-Access Stratum (NAS) message carrying a small data packet (e.g., Internet Protocol (IP), non-IP, short message service (SMS)). This uplink NAS message is delivered to the C-SGN via the CIoT Base Station (CIoT BS). This uplink NAS message is transmitted on a Signaling Radio Bearer (SRB). Thus, a setup of a Data Radio Bearer (DRB) is not required. Further, Access Stratum (AS) Security may be omitted.
The C-SGN decrypts the uplink NAS message to obtain the small data packet. The C-SGN forwards the small data packet according to the data type of the small data packet. For IP small data, the C-SGN sends it on the SGi interface. For SMS, the C-SGN sends it to an entity related to SMS (e.g., SMS Gateway Mobile Services Switching Center (SMS-GMSC), SMS Interworking Mobile Services Switching Center (SMS-IWMSC), SMS router). For Non-IP small data, the C-SGN sends it to the Service Capability Exposure Function (SCEF).
In the Mobile Terminated (MT) small data transmission for the non-roaming case, the C-SGN transmits a downlink NAS message carrying a small data packet to the UE through the CIoT BS. Also for the small data packet transmission in downlink, a DRB is not required and AS security may be omitted.
The CIoT BS shown in FIG. 1 is a base station in the CIoT Radio Access Network (CIoT RAN). An LTE eNB configured to be connected to the C-SGN may be used instead of the CIoT BS in FIG. 1. This LTE eNB may be an eNB that supports LTE eMTC.
Meanwhile, the architecture according to the solution 18 provides infrequent small data transmission on the user plane. The architecture according to the solution 18 has the feature of reusing information obtained from the previous RRC connection for the subsequent RRC connection setup, thereby reducing the signaling required for UE Radio Resource Control (RRC) state transition.
Specifically, a UE enters the RRC-Idle mode from the RRC-Connected mode and retains information about the RRC connection (e.g., an Access Stratum Security Context, bearer related information (incl. RoHC state information), and L2/1 parameters when applicable) while it is in RRC-Idle mode. Similarly, an eNB retains information about the RRC connection of the UE (e.g., Access Stratum Security Context, bearer-related information (incl. RoHC state information), and L2/1 parameters when applicable). Further, the eNB and MME retain S1AP UE Contexts. Furthermore, the eNB retains S1-U tunnel addresses.
When returning to the RRC-Connected mode, the UE sends an RRC Connection Resume Request to the eNB. The eNB restores a DRB(s), a security context, an S1AP connection, and an S1-U tunnel(s) based on the previously retained information about the RRC connection. Further, the eNB informs the MME of a UE state change using a new S1AP message (e.g., S1AP: UE Context Resume Request). The MME returns the Evolved Packet System (EPS) Connection Management (ECM) state of the UE to the ECM-Connected state and then sends a Modify Bearer Request message to the S-GW. As a result, the S-GW recognizes that the UE is in the connected state and hence becomes ready to transmit downlink data towards the UE.
In the solution 18, the UE can return to RRC-Connected and ECM-Connected without transmitting a NAS message (i.e., Service Request). Further, as compared with the legacy RRC connection setup procedure, the following RRC messages can be removed:                RRC Connection Setup Complete;        RRC Security Mode Command;        RRC Security Mode Complete;        RRC Connection Reconfiguration; and        RRC Connection Reconfiguration Complete.        
The above-described solution 2 and solution 18 are also referred to as “Data over NAS (DoNAS)” and “AS context caching”, respectively. Alternatively, the solution 2 and solution 18 are also referred to as “Control Plane CIoT EPS optimisation” and “User Plane CIoT EPS optimisation”, respectively.
At this time, it is assumed that the solution 2 does not use Access Stratum (AS) security and PDCP, and neither the solution 2 nor the solution 18 uses the SRB 2.
In some implementations, the solution to be applied to the UE may be selected by the core network (i.e., MME, C-SGN) in the attach procedure for this UE. Alternatively, in some implementations, the UE itself may select the solution. The core network or the UE makes an initial selection of the solution to be applied to the UE, and after that the core network or the UE changes the selected solution.
Non Patent Literature 2 describes that the UE may determine, during the attach procedure, which of the Solution 2 architecture and the Solution 18 architecture it would prefer to use. Further, Non Patent Literature 2 describes that an AS procedure or a NAS procedure may include information for allowing the network to select the solution 2 or the solution 18 for data transmission.
Non Patent Literature 3 describes that the UE may include a “Preferred Network Behaviour” indication in a NAS message such as an Attach Request, a PDN Connection Request, and a Tracking Area Update (TAU) Request. The Preferred Network Behaviour indicates a solution that the UE can support or that the UE would prefer to use. Specifically, the Preferred Network Behaviour may include the following information:                Whether Control Plane CIoT EPS optimisation is supported;        Whether User Plane CIoT EPS optimisation is supported;        Whether Control Plane CIoT EPS optimisation is preferred or whether User Plane CIoT EPS optimisation is preferred;        Whether S1-U data transfer is supported;        Whether SMS transfer without Combined Attach is requested; and        Whether Attach without PDN Connectivity is supported.        